Up to now, steel sheets 590 MPa or less in tensile strength standard have generally been used for parts mostly composing the body of an automobile or a motorcycle.
In recent years, studies have been conducted for enhancing a material strength to a large extent and the application of further enhanced high-strength steel sheets is being attempted with the aim of the reduction of a car body weight for the improvement of fuel efficiency and the improvement of collision safety.
High-strength steel sheets produced for the fulfillment of the aforementioned objects are mostly used for car body frame members and reinforcement members, seat frame parts and others of an automobile or a motorcycle and a steel sheet 780 MPa or more in tensile strength of the base steel having excellent formability is strongly in demand.
Such parts are subjected to working such as press forming and roll forming. However, due to requirements from car body designers and other industrial designers, it is sometimes difficult to drastically change the shapes of such parts from the shapes to which a conventional steel sheet 590 MPa or less in tensile strength is applicable and therefore, for facilitating the forming of a complicated shape, a high-strength steel sheet having excellent workability is required.
In the meantime, working methods are shifting from conventional drawing with a blank holder to simple stamping or bend working in accordance with the adoption of a higher-strength steel sheet. In particular, when a bend ridge curves in the shape of a circular arc or the like, sometimes the ends of a steel sheet are elongated, in other words, stretched flange working is applied. Further, to some parts, burring working wherein a flange is formed by expanding a working hole (lower hole) is often applied. In some large expansion cases, the diameter of the lower hole is expanded up to 1.6 times or more. Meanwhile, an elastic recovery phenomenon after the working of a part, such as spring back, tends to appear as the strength of a steel sheet increases and hinders the accuracy of the part from being secured. For that reason, contrivances, for example to reduce a inner radius for bending up to about 0.5 mm in bend working, are often employed in plastic working methods.
However, in such working, though a steel sheet is required to have local formability such as stretched flange formability, hole expandability, bendability and the like, a conventional high-strength steel sheet is insufficient in securing such formability, and therefore, the problem of a conventional high-strength steel sheet has been that troubles, including cracks, occur and a product cannot be processed stably.
In the meantime, such press-formed parts are very often joined with other parts by spot welding or other welding. However, in the case of a high-strength steel sheet 780 MPa or more in tensile strength in general, a metallurgical method such as the increase of a C-content in steel is often adopted as a means effective for securing strength and the problem caused by the adoption of such a method has been that a weld metal is hardened extremely by heating and cooling at the time of welding and therefore the properties of a weld and the functions of a product are deteriorated.
A hitherto reported high-strength steel sheet having improved the stretched flange formability is the one proposed by Japanese Unexamined Patent Publication No. H9-67645. However, the technology merely improves the stretched flange formability after shearing and does not necessarily improve the properties of a weld.
Further, Japanese Examined Patent Publication Nos. H2-1894 and H5-72460 propose methods for improving weldability of a high-strength steel sheet. The former technology improves the cold-workability and weldability of a high-strength steel sheet. However, with regard to the improvement of cold-workability cited in the technology, the improvement of local formability such as stretched flange formability, hole expandability, bendability and the like is not confirmed sufficiently. In contrast, the latter technology proposes the improvement of stretched flange formability in addition to weldability. However, the strength of a steel sheet included in the invention is at the level of about 550 MPa and the technology is not the one that deals with a high-strength steel sheet 780 MPa or more in tensile strength.
Furthermore, as a result of earnest studies by the present inventors, the following findings have been obtained. In the case of a high-strength steel sheet 780 MPa or more in tensile strength of the base steel, the main strengthening mechanism is actuated mostly by hard martensite and bainite in the second phase and a C content in steel functions as a major factor in the strengthening mechanism. However, as a C content increases, local formability is likely to deteriorate and, at the same time, the hardness of a weld increases conspicuously. Nevertheless, with regard to the aforementioned problems of a high-strength steel sheet 780 MPa or more in tensile strength of the base steel, no proposal focused on the improvement of local formability and the suppression of weld hardening can be found.